If I found the money alone, I'd keep it. If I was with someone, I'd turn it in, because I just know it would turn out bad.
Hell, I've kept important secrets for over 30 years, without even coming close to telling anyone.
I know things about people that they don't have a clue that I know, and have no intention of ever divulging the information to anyone.
It runs in the family. My sister is the same way. When she was starting out with Ma Bell, she was a service rep for a time. It was her job to listen to conversations and evaluate service. She heard some juicy stuff and some bad stuff, and would not even hint as to the nature. And this was back in the early 70's and she still refuses to discuss it. I once asked her what she would do if she overheard someone planning a murder. She just looked away, and refused to comment.
A Simple Plan is a good movie. You just know from the beginning that the plan is doomed.